Conventionally, research and development of batteries having both a high output voltage and a high energy density have been extensively advanced. In particular, secondary cells having a low internal resistance, accompanied by less lowering of cell capacity due to charge and discharge, and that are superior in charge and discharge cycle characteristics have been demanded. For example, a lithium secondary cell in which microcrystalline silicon or amorphous silicon in a state of a thin film is used as a negative electrode material (negative electrode active material) has been known (see, Patent Document 1). Specifically, a lithium secondary cell in which a negative electrode is used that includes a negative electrode material layer composed of a silicon thin film formed on a collector is disclosed. For forming the silicon thin film, thin film formation techniques such as CVD methods (may be also referred to as “chemical vapor-phase growth method”, or “chemical vapor deposition method”) and sputtering methods have been used.
In such techniques, materials such as silicon are considered to repeatedly expanded/contracted as lithium is stored/released. Since adhesion between the collector and the negative electrode material layer is high in the negative electrode in which a silicon film is formed on the collector, the expansion/contraction of the collector occurs frequently with the expansion/contraction of the negative electrode material. Therefore, charging and discharging may be accompanied by irreversible deformation such as shriveling of the negative electrode material layer and the collector. In particular, when a metal foil that is highly ductile such as copper foil is employed as a collector, the degree of deformation tends to increase. When the negative electrode deformes, the energy density of the battery may decrease due to an increase in electrode volume, thereby producing a heterogeneous electrochemical reaction. In addition, while the expansion/contraction repeats due to charging and discharging, the negative electrode material may be pulverized and become detached from the collector, or may become detached while maintaining the form of a thin film in some cases. Accordingly, the charge and discharge cycle characteristics of the battery may deteriorate.
An exemplary method for inhibiting the deformation of the negative electrode includes a method in which a material having superior mechanical strength such as high tensile strength and elastic modulus in tension is used as a collector. However, when a negative electrode material layer composed of a negative electrode material in the form of a thin film is formed on a collector composed of such a material, the adhesion between the collector and the negative electrode material layer may be insufficient, whereby satisfactory charge and discharge cycle characteristics may not be achieved. Therefore, Patent Document 1 discloses a technique to inhibit occurrence of shriveling and the like while inhibiting detachment of the negative electrode material during charging and discharging by providing a middle layer composed of a material that alloys with the negative electrode material, and using a collector having a mechanical strength greater than that of the middle layer. Specifically, a copper layer is used as the middle layer, and nickel foil is used as the collector.
In addition to Patent Document 1 as described above, a technique is disclosed in which a thin film formed by a solid solution of copper on silicon is used as a negative electrode material layer to control the quantity of storage of lithium, thereby inhibiting expansion of the negative electrode material in the case of storage of lithium (see, Patent Document 2). Furthermore, a technique is disclosed in which an alloy thin film composed of a metal that alloys with lithium, and a metal that does not alloy with lithium are used to control the quantity of storage of lithium, thereby inhibiting expansion of the negative electrode material in the case of storage of lithium (see, Patent Document 3). Specifically, as a metal that forms a solid solution or an intermetallic compound by alloying with lithium, Sn, Ge, Al, In, Mg, Si or the like is used, while Cu, Fe, Ni, Co, Mo, W, Ta, Mn or the like is used as the metal that does not alloy with lithium.
Additionally, a technique is disclosed in which a collector having deformed parts where a deformation amount of 5 μm to 20 μm in a depthwise direction are formed in an amount of 10 or more per cm2, and having an opening ratio of no greater than 4% provided due to the presence of the deformed parts is used, whereby deformation of the electrode resulting from charging and discharging is inhibited (see, Patent Document 4). Moreover, a technique in which a material having no storability of lithium is provided on at least one of the surfaces and the interior of a negative electrode material layer in the form of a thin film which can store/release reversibly lithium is disclosed (see, Patent Document 5).    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2002-083594.    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2002-289177.    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2002-373647.    [Patent Document 4] Japanese Unexamined Patent Application Publication No. 2003-017069.    [Patent Document 5] Japanese Unexamined Patent Application Publication No. 2005-196971.